Coherent Receiver With Mixed Signal Processing

ABSTRACT

A receiver system is provided for receiving a coherent Pulse Amplitude Modulation (PAM) encoded signal. The receiver system may include an optical polarization component configured to modulate a polarization of the received coherent PAM encoded signal. The receiver system may further include a digital signal processor (DSP) configured to perform polarization recovery between the received coherent PAM encoded signal and the LO signal using a first control loop, and to perform phase recovery between the received coherent PAM encoded signal and the LO signal using a second control loop.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 17/206,854, filed on Mar. 19, 2021, which is a divisional of U.S.patent application Ser. No. 16/675,375, filed on Nov. 6, 2019, thedisclosures of which are incorporated herein by reference.

BACKGROUND

In an optical network, a transmitter system may modulate light signalsto encode data, and a receiver system may detect and decode themodulated light signals to recover the data. AnIntensity-Modulation/Direct-Detection (IM-DD) Pulse Amplitude Modulation(PAM) system modulates intensity of a carrier signal to encode data. Incontrast, a coherent PAM system modulates amplitude of the optical fieldof a carrier signal to encode data. A coherent Quadrature AmplitudeModulation (QAM) system modulates the optical field of two carriersignals to encode data, such as either or both amplitude and/or phase ofthe two carrier signals.

In long-haul and metro networks, QAM systems are widely used. However,for low-power datacenter-reach optical interconnects, QAM technology hasseveral challenges. First, QAM transceivers may not be compatible withIM-DD PAM transceivers, but interoperability is often required indatacenters, as datacenter networks are often upgraded block by block.Second, QAM systems may use more power than PAM systems since QAMtransceivers may require higher bias voltage for modulation, andadditional components such as additional Digital Signal Processing (DSP)functional blocks may be needed for polarization and phase recovery.Third, to perform phase modulation, QAM systems may have stricterrequirements on the lasers and modulators used, such as speed,bandwidth, extinction ratio etc.

Existing polarization-diverse and phase-diverse receiver systems canreceive and decode coherent encoded signals, such as coherent QAM orcoherent PAM encoded signals. Since properties of a signal can changeduring transmission, the receiver system needs to perform recovery ofthese properties once the signal is received. For example, a receivedcoherent PAM encoded signal may be split by a Polarizing Beam Splitter(PBS) into two beams with orthogonal polarizations, such as thetransverse electric (TE) mode polarized light beam and the transversemagnetic (TM) mode polarized light beam. A 90° polarization rotator maybe built into the PBS to convert the TM mode polarized light beam intothe TE mode polarized such that the two outputs of the PBS are bothTE-mode polarized light beams. For convenience, through this document,the principal polarization axes of the PBS are denoted as x-axis andy-axis. Further, the original TE-mode polarized signal is denoted as“x-polarized beam,” while the TE-mode polarized signal converted fromthe original TM mode signal is denoted as “y-polarized beam.”

Recovery of polarization and phase information is then performed on eachof the polarized beams by reference to a signal from a Local Oscillator(LO) with known properties. In this regard, an Optical Coupler (OC)splits the LO signal into two beams, and interferences may then beperformed between each polarized coherent PAM encoded beam and one ofthe two LO beams by two respective hybrids. For instance, an x-polarized90° hybrid may perform interference between the x-polarized beam and afirst LO beam, and a y-polarized 90° hybrid may perform interferencebetween the y-polarized beam and a second LO beam. Since properties ofthe LO are known, the resultant wave from each interference may beanalyzed to determine properties of each polarized coherent PAM encodedbeam.

As a result of the interference, the two hybrids may output foursignals—two phases for each of the two polarizations. These four signalsare then respectively passed through four photodetectors (PD) thatconvert optical signals to electrical signals, and four transimpedanceamplifiers (TIA) that amplify the electrical signals. In some examples,each of the four PDs may be a pair of balanced PDs configured to cancelcommon-node noises. The amplified electrical signals are thenrespectively passed through four analog to digital converters (ADC) andconverted into digital signals. Once the four separated signals areconverted from optical into electrical by the PDs/TIAs, and digitized bythe ADCs, the Digital Signal Processor (DSP) may extract amplitude andphase information in the electrical domain to decode and recover thedata.

Although a coherent PAM system may use a conventional polarization- andphase-diverse digital coherent receiver system, such a complex receiversystem may be inefficient for recovering coherent PAM encoded signals.Further, the conventional polarization-diverse and phase-diversereceiver system is not compatible with an IM-DD PAM transmitter system.

SUMMARY

The present disclosure provides for receiver system comprising anoptical polarization component and a digital signal processor (DSP). Theoptical polarization component may be configured to modulate apolarization of a received coherent Pulse Amplitude Modulation (PAM)encoded signal. The DSP may be configured to perform polarizationrecovery between the received coherent PAM encoded signal and a localoscillator (LO) signal using a first control loop; and to perform phaserecovery between the received coherent PAM encoded signal and the LOsignal using a second control loop.

The DSP may be configured to determine polarization adjustments to thereceived coherent PAM encoded signal in an electrical domain and theoptical polarization component may be configured to apply thepolarization adjustments to the received coherent PAM encoded signal inan optical domain. The DSP may be configured to determine thepolarization adjustments by maximizing a beating signal power betweenthe LO signal and a polarized portion of the received coherent PAMencoded signal with encoded data. The receiver system may furthercomprise a low-speed circuit configured to detect an average power of apolarized portion of the received coherent PAM encoded signal, whereinthe DSP may be configured to determine the polarization adjustments byminimizing the average power of the polarized portion.

The optical polarization component may be an optical polarizationcontroller for receiving a single polarization or dual polarizationcoherent PAM encoded signal, or an optical demultiplexer for receiving adual-polarization coherent PAM encoded signal.

The receiver system may further comprise one or more optical phasemodulators, wherein the DSP may be configured to determine phaseadjustments to the LO in an electrical domain and the one or moreoptical phase modulators may be configured to apply the phaseadjustments to the LO signal in an optical domain. The DSP may beconfigured to determine the phase adjustments based on known pilot datasymbols inserted in the received coherent PAM encoded signal. The DSPmay be configured to determine frequency adjustments to the LO signal inan electrical domain and the one or more optical phase modulators may beconfigured to apply the frequency adjustments to the LO signal in anoptical domain.

The DSP may be configured to determine frequency adjustments to the LOin an electrical domain and the LO may be configured to apply thefrequency adjustments to the LO signal in an optical domain.

The receiver system may further comprise a wavelength demultiplexerconfigured to receive coherent PAM encoded signal that includes aplurality of wavelengths.

The first control loop may be a polarization feedback control loop, andthe second control loop may be a phase feedback control loop.

The first control loop and the second control loop may be part of a samecontrol loop for joint polarization and phase recovery. The control loopfor joint polarization and phase recovery may be configured to introducea 45° polarization angle between the received signal and the LO. Thereceiver system may further comprise a joint polarization-diverse andphase-diverse coherent detection circuitry, wherein, thepolarization-diverse coherent detection circuitry and the phase-diversecoherent detection circuitry may share one or more common circuitelements.

The receiver system may further comprise a two-mode switch, wherein,when the two-mode switch is set in a first mode, the two-mode switch maybe configured as an optical coupler (OC) for receiving coherent PAMencoded signal, and when the two-mode switch is set in a second mode,the two-mode switch may be configured as a switch for receiving anintensity modulation direct-detect (IM-DD) PAM encoded signal.

The present disclosure further provides for a receiver system comprisinga polarization beam splitter, two hybrids, an analog equalizer, and aDSP. The polarization beam splitter may be configured to split areceived coherent PAM encoded signal into two polarized PAM encodedsignals. The two hybrids may be configured to combine each of the twopolarized PAM encoded signals with a LO signal. The analog equalizer maybe configured to perform polarization recovery between the receivedcoherent PAM encoded signal and the LO signal, and perform phaserecovery between the received coherent PAM encoded signal and the LOsignal. The DSP may be configured to provide equalizer coefficients tothe analog equalizer for polarization and phase recovery. The analogequalizer may be a 4×2 multi-input multi-output equalizer.

The present disclosure still further provides for receiving, by areceiver system, a coherent PAM encoded signal; receiving, by thereceiver system, a LO signal; performing, by the receiver system usingmixed signal processing, polarization recovery between the receivedcoherent PAM encoded signal and the LO signal using a first controlloop; and performing, by the receiver system using mixed signalprocessing, phase recovery between the received coherent PAM encodedsignal and the LO signal using a second control loop.

The mixed signal processing may include processing in an optical domainand processing in an electrical domain. The mixed signal processing mayinclude processing in an analog domain and processing in a digitaldomain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show example receiver systems with mixed electrical andoptical signal processing for single polarization signals in accordancewith aspects of the disclosure.

FIGS. 2A and 2B show example receiver systems with mixed electrical andoptical signal processing for dual polarization signals in accordancewith aspects of the disclosure.

FIGS. 3A and 3B show example receiver systems with mixed electrical andoptical signal processing for signals with multiple wavelengths inaccordance with aspects of the disclosure.

FIGS. 4A and 4B show example receiver system with mixed electrical andoptical signal processing with joint polarization-diverse andphase-diverse coherent detection in accordance with aspects of thedisclosure.

FIGS. 5A, 5B, and 5C show example 1×2 optical polarization controllersthat can be used to enable joint polarization-diverse and phase-diversecoherent detection in accordance with aspects of the disclosure.

FIG. 6 shows an example receiver system with mixed analog and digitalsignal processing in accordance with aspects of the disclosure.

FIGS. 7A and 7B show example transmission systems with mixed signalprocessing in accordance with aspects of the disclosure.

FIG. 8 is a block diagram showing an example receiver system inaccordance with aspects of the disclosure.

FIG. 9 is a flow diagram illustrating an example method in accordancewith aspects of the disclosure.

DETAILED DESCRIPTION Overview

The technology relates generally to a coherent receiver system withmixed signal processing. A PAM system may be used for datacenter-reachoptical interconnects. In terms of interoperability, a coherent PAMtransmitter and an IM-DD PAM transmitter may both use the same type ofmodulators, such as Mach-Zehnder Modulators (MZM). A receiver system isprovided with mixed signal processing. For instance, the receiver systemmay include an optical polarization component configured to modulate apolarization of the received coherent PAM encoded signal. The receiversystem may further include a digital signal processor (DSP) configuredto perform polarization recovery between the received coherent PAMencoded signal and the LO signal using a first control circuitry orloop, and to perform phase recovery between the received coherent PAMencoded signal and the LO signal using a second control circuitry orloop.

With respect to polarization recovery, polarization adjustments to bemade may be determined by the DSP in the electrical domain, and theoptical polarization controller may make these polarization adjustmentsto the received coherent PAM encoded signal in the optical domain. Forexample, the DSP may determine polarization adjustments in a feedbackcontrol loop by maximizing a beating signal power between the LO and apolarized portion of the coherent PAM encoded signal with encoded data.Alternatively, the DSP may determine polarization adjustments in afeedback control loop by minimizing power of a polarized portion withoutencoded data.

With respect to phase recovery, phase adjustments to be made may bedetermined by the DSP in the electrical domain, and one or more opticalphase modulator may make these phase adjustments to the LO signal in theoptical domain. For example, the DSP may determine phase adjustments ina feedback control loop based on known pilot data symbols inserted inthe coherent PAM encoded signal.

In some instances, instead of independent polarization and phaserecovery control loops or circuitry, polarization and phase recovery maybe performed using a single control loop or circuitry through a jointpolarization-diverse and phase-diverse coherent detection method. Inthis regard, the optical polarization component may be implemented usinga 1×2 optical polarization controller. The 1×2 optical polarizationcontroller may be configured to apply polarization adjustments to thecoherent PAM encoded signal to introduce a 45° of polarization anglebetween the adjusted PAM encoded signal and the LO. Interference may beperformed between the adjusted coherent PAM encoded signal and twophase-diverse LO signals. The DSP may then analyze the resulting beamsand known pilot data symbols to determine adjustments for polarizationand phase recovery.

The receiver system may be configured with additional and/or alternativefeatures for receiving different types of PAM encoded signals. Forinstance, to receive a dual-polarization coherent PAM encoded signal, anoptical polarization demultiplexer may be used as the opticalpolarization component to separate the two encoded polarizations. Asanother example, to receive a PAM encoded signal with multiplewavelengths, a wavelength demultiplexer may be provided to separate themultiple wavelengths into different beams for separate analysis.

Further, the receiver system may be configured to receive an intensitymodulation direct-detect (IM-DD) PAM encoded signal as well as acoherent PAM encoded signal. In this regard, a two-mode switch may beprovided such that, when the two-mode switch is set in a first mode, thetwo-mode switch is configured as an optical coupler (OC) for receiving acoherent PAM encoded signal. In contrast, when the two-mode switch isset in a second mode, the two-mode switch is configured as a switch forreceiving an IM-DD PAM encoded signal.

Additionally or alternatively, a coherent receiver system may beconfigured with mixed analog and digital processing. For instance, thereceiver system may include a polarization beam splitter (PBS)configured to split a PAM encoded signal into two polarized PAM encodedsignals, and two hybrids configured to combine each of the two polarizedPAM encoded signals with a local oscillator (LO) signal. The receiversystem may further include an analog equalizer and a DSP configured toperform polarization and phase recovery between the received coherentPAM encoded signal and the LO signal.

In another aspect, a PAM transmission system may be provided withinteroperability between coherent PAM and IM-DD PAM transmission. Inthis regard, the PAM transmission may include a transmitter system and areceiver system. When set in a first mode, the transmitter system may beconfigured to transmit coherent PAM encoded data, and the receiversystem may be configured to receive the coherent PAM encoded data. Whenset in a second mode, the transmitter system may be configured totransmit IM-DD PAM encoded data, and the receiver system may beconfigured to receive the IM-DD PAM encoded data. In this regard, atwo-mode switch may be provided in both the transmitter system and thereceiver system to allow the change between the coherent PAM and IM-DDPAM modes.

The technology provides power efficient receiver systems in datacenters.The technology provides receiver systems that are compatible with bothcoherent PAM and IM-DD PAM transmission systems, which increases designflexibility of a datacenter. Further, by using mixed optical andelectrical processing, and/or mixed electrical and digital processing,components with high power consumption may be reduced. Additionally, byusing a PAM system that does not perform phase modulation, requirementson optical equipment, such as lasers, may be relaxed, which may furtherincrease power efficiency and design flexibility.

Example Systems

An efficient coherent PAM receiver system may be provided with mixedsignal processing capabilities. As mentioned above, to receive anddecode a one-dimensional signal, such as a coherent PAM encoded signal,a polarization-diverse and phase-diverse receiver system may beinefficient. First, a coherent PAM encoded signal uses only one carriersignal and do not include phase modulation, thus it is unnecessary toseparate multiple phases. Further, where the coherent PAM encoded signalincludes only a single polarization, it is also unnecessary to separatemultiple polarizations. Accordingly, FIGS. 1A-4 show example receiversystems that use mixed optical and electrical signal processing. FIG. 6shows example receiver system with mixed analog and digital processing.FIGS. 1A-3B show receiver systems where polarization and phase recoveryare performed using two independent control circuitries or loops, whileFIGS. 4A and 4B shows a receiver system where polarization and phaserecovery are performed using a single control circuitry or loop througha joint polarization-diverse and phase-diverse coherent detectionmethod. Each of these receiver systems may include one or more frequencycontrol circuitry or loops for coarse and/or fine adjustments. FIGS. 5A,5B, and 5C show example implementations of an optical polarizationcomponent that can be used in the joint polarization-diverse andphase-diverse coherent detection method.

Referring to FIG. 1A, an example coherent PAM receiver system 100A forreceiving a single polarization coherent PAM encoded signal is shown.The receiver system 100A may receive an incoming signal from atransmitter system (not shown). For instance, a transmitter system mayhave generated a coherent carrier signal, and encoded with the carriersignal with data using a data modulator. For PAM encoding, the datamodulator may be a high-speed optical modulator, such as a Mach-ZehnderModulator (MZM). The data modulator may encode data by convertinghigh-speed electrical data signal into high-speed optical data signaland add to the carrier signal. In this example, data is encoded in onlyone polarization, thus resulting in a single polarization coherent PAMencoded signal. Although the transverse electric (TE) mode polarizedlight beam, denoted as x-polarization, is shown in FIG. 1A as thepolarization with encoded data, in other examples data may be encoded inany other polarization. The coherent PAM encoded signal may then betransmitted by the transmitter system to the receiver system 100A, suchas through one or more optical fibers. While in transmission, variousproperties of the coherent PAM encoded signal, such as polarization,phase, and/or frequency, may change randomly.

To recover properties of the received single polarization coherent PAMencoded signal that may have changed during transmission, the receiversystem 100A may also receive a signal from a Local Oscillator (LO) 120as reference. An optical coupler (OC) 130 mixes the received coherentPAM encoded signal and the LO signal. For example, an opticalpolarization component, such as an optical polarization controller 110,may initially select x-polarized component of the coherent PAM encodedsignal for passing through the OC 130, and an optical phase modulator140 may initially allow the LO signal to pass through the OC 130 withoutmodification to the phase or frequency. During the recovery process, theoptical polarization controller 110 and the optical phase modulator 140may change the polarization, frequency, and/or phase of the signals viafeedback control loops with one or more processors, such as coherent PAMdigital signal processor (DSP) 180, as described further below.

To be analyzed by the DSP 180, which is in the electrical domain, themixed LO signal and the received single polarization coherent PAMencoded signal are converted into electrical and/or digital domains. Forinstance as shown, the mixed LO and coherent PAM encoded signal arepassed through a photodetector (PD) 150 for converting from optical toelectrical domain. In some instances the PD 150 may be a pair ofbalanced PDs configured to cancel common-mode noises. For example, themixed signal may be used as input to each of the two PDs (shown as 2solid lines), and the output of the two PDs may be combined into oneelectrical signal that cancels out the common-mode noises (shown as 1dashed line). The electrical signal may be amplified by thetransimpedance amplifier (TIA) 160. The amplified electrical signal maythen be digitized by an analog to digital converter (ADC) 170. Thedigitized signal may be passed to the coherent PAM digital signalprocessor (DSP) 180 for analyses. As such, the configuration of receiversystem 100A reduces the number of PD, TIA, and ADCs required by a factorof four as compared to a conventional polarization-diverse andphase-diverse receiver system.

One or more feedback control loops may be used between the DSP 180 andthe optical polarization controller 110, the LO 120, and/or the opticalphase modulator 140 in order to match properties of the LO signal andthe received single polarization coherent PAM encoded signal. The DSP180 may be one or more processors of any of a number of types, such asprocessors 820 of FIG. 8. The DSP 180 may determine adjustments to bemade in the electrical domain, while the optical polarization controller110, the LO 120, and/or the optical phase modulator 140 may make theseadjustments in the optical domain. By analyzing effects of theadjustments, the DSP 180 may determine further adjustments to be made,until the DSP 180 determines that the properties of the LO signalsufficiently match the properties of the coherent PAM encoded signal.These feedback control loops are thus implemented by mixed optical andelectrical components. The feedback control loops may be used to performpolarization, phase, and/or frequency recovery.

With respect to polarization recovery, a polarization feedback controlloop 192 between the DSP 180 and the optical polarization controller 110may be used. Polarization adjustments to be made may be determined bythe DSP 180 in the electrical domain, and the optical polarizationcontroller 110 may make these polarization adjustments to the receivedcoherent PAM encoded signal in the optical domain. The opticalpolarization controller 110 may be an optical signal processorconfigured to change polarization of a coherent PAM encoded signal. Asan example, the optical polarization controller 110 may be implementedusing interferometers with variable phase shifters. The opticalpolarization controller 110 shown in FIG. 1A is a 1×1 opticalpolarization controller (1 input, 1 output). The DSP 180 may determinepolarization adjustments to be made by analyzing beating signal powerbetween the LO signal and the received single polarization coherent PAMencoded signal. Since the maximum achievable beating signal powerbetween two signals may be achieved when the two signals have the samepolarization, the DSP 180 may determine the polarization adjustments tobe made by maximizing beating signal power between the LO signal and thex-polarized coherent PAM encoded signal. For instance, initially the DSP180 may receive the LO signal and the x-polarized coherent PAM encodedsignal, and determine an initial beating signal power. The DSP 180 mayincrease the polarization control coefficients, for example the requiredphase shifter control voltages, by a predetermined amount and instructthe optical polarization controller 110, via polarization feedbackcontrol loop 192, to adjust polarization of the coherent PAM encodedsignal by the predetermined amount. Since the maximum beating signalpower may not be known by the receiver system at the starting stage,multiple back and forth adjustments are usually needed until adjustmentsin either direction would decrease the power. After the adjustment, theDSP 180 may continue to receive LO signal and x-polarized coherent PAMencoded signal, and determine beating signal power. If the DSP 180determines that, after the adjustment, the beating signal power betweenthe LO signal and the x-polarized coherent PAM encoded signal increaseddue to the increased polarization control coefficients, the DSP 180 mayfurther increase the polarization control coefficient, and the opticalpolarization controller 110 may apply this adjustment. Conversely, ifthe DSP 180 determines that the beating signal power between the LOsignal and the x-polarized coherent PAM encoded signal decreased due tothe increased polarization control coefficient, the DSP 180 may decreasethe polarization control coefficient, and the optical polarizationcontroller 110 may apply this adjustment. Further adjustments may bedetermined and applied until the beating signal power is maximized, forexample to a predetermined threshold.

With respect to carrier frequency and/or phase recovery, one or morefrequency and/or phase feedback control loops may be used between theDSP 180 and the LO 120 and/or optical phase modulator 140. Phase and/orfrequency adjustments to be made may be determined by the DSP 180 in theelectrical domain, and implemented by the LO 120 and/or the opticalphase modulator 140 in the optical domain. In comparison to thehigh-speed data modulator, such as an MZM, that encodes data in thetransmitter system, the optical phase modulator 140 in the receiversystem 100A may be a low-speed optical modulator. Since laser phasechanges for the carrier signal is much slower than the high-speedcoherent PAM encoded signal, the required bandwidth for the opticalphase modulator 140 may also be significantly lower than the requiredbandwidth for the data modulator in the transmitter system. By using alow-speed, low-bandwidth modulator as the optical phase modulator 140,the receiver system 100A may thus reduce power consumption.

The DSP 180 may determine the frequency adjustments to be made usingfast Fourier transforms (FFT). FFT may also be a low-speed operationsince laser frequency for the carrier signal typically changes veryslowly compared to data rate of the coherent PAM encoded signal. Assuch, the DSP 180 may reduce power consumption by performing thelow-speed operation, as compared to the high-speed operations involvingthe hybrids of receiver system 100. Based on the FFT, frequency offsetbetween LO signal and the coherent PAM encoded signal may be estimated.Frequency adjustments may then be determined based on the frequencyoffset and applied via feedback control loop 194 and/or 196.

For instance, initially the DSP 180 may receive the LO signal and thecoherent PAM encoded signal, and use FFT to determine an initialfrequency offset between the LO signal and the coherent PAM encodedsignal. The DSP 180 may increase the frequency control coefficient, forexample the required temperature control voltage or laser drive currentfor the LO, by a predetermined amount and instruct the LO 120, viafrequency feedback control loop 194, to adjust frequency based on thepredetermined amount. Additionally or alternatively, the DSP 180 mayinstruct the optical phase modulator 140 via frequency/phase feedbackcontrol loop 196, to adjust frequency based on the predetermined amount.For example, coarse frequency adjustments may be made via frequencyfeedback control loop 194, while fine frequency adjustments may be madevia feedback control loop 196. After the adjustment, the DSP 180 maycontinue to receive LO signal and coherent PAM encoded signal and useFFT to determine frequency offset. If the DSP 180 determines that, afterthe adjustment, the frequency offset between the LO signal and thesingle polarization coherent PAM encoded signal decreased due to theincreased frequency control coefficient, the DSP 180 may furtherincrease the frequency control coefficient, and the LO 120 and/oroptical phase modulator 140 may apply this adjustment. Conversely, ifthe DSP 180 determines that the frequency offset between the LO signaland the single polarization coherent PAM encoded signal increased due tothe increased frequency control coefficient, the DSP 180 may decreasethe frequency control coefficient, and the LO 120 and/or optical phasemodulator 140 may apply this adjustment. Further adjustments may bedetermined and applied until the frequency offset is minimized, forexample to a predetermined threshold.

The DSP 180 may determine phase adjustments to be made using pilot datasymbols. Pilot symbols are special data symbols which are known to areceiver system. Pilot symbols may be inserted by a transmitter systeminto the regular data symbols of an optical signal, such as the coherentPAM encoded signal, through time-division multiplexing. For example, thepilot symbols may be inserted in fixed time slots. For instance,initially a maximum achievable pilot symbol signal level, whichrepresents the closest phase match between LO 120 and received coherentPAM encoded signal, may be measured and calibrated. This may beperformed using training data with random LO phase change through theoptical phase modulator 140. Once the maximum achievable pilot symbolsignal level is calibrated, known pilot data symbols may be sentperiodically by a transmitter system via the coherent PAM encoded signalto receiver system 100A. Based on the pilot symbol signal level of theseknown pilot data symbols, phase offset between LO signal and thecoherent PAM encoded signal may be estimated, based on which adjustmentsmay be determined and applied in phase feedback control loop 196.

For instance, initially the DSP 180 may receive the LO signal and pilotsymbols encoded in the coherent PAM encoded signal, and determine aninitial phase offset between the LO signal and the coherent PAM encodedsignal based on the pilot symbol signal level. The DSP 180 may thenincrease the phase control coefficient, for example the required drivevoltage for the optical Phase Modulator 140, by a predetermined amountand instruct the optical phase modulator 140, via phase feedback controlloop 196, to adjust phase by the predetermined amount. After theadjustment, the DSP 180 may continue to receive LO signal and pilotsymbols in the coherent PAM encoded signal, and determine phase offsetbased on the pilot symbol signal level. If the DSP 180 determines that,after the adjustment, the phase offset between the LO signal and thesingle polarization coherent PAM encoded signal decreased due to theincreased phase control coefficient, the DSP may further increase thephase control coefficient, and the LO 120 and/or optical phase modulator240 may apply this adjustment. Conversely, if the DSP 180 determinesthat the phase offset between the LO signal and the coherent PAM encodedsignal increased due to the increased the phase control coefficient, theDSP may decrease the phase control coefficient, and the LO 120 and/oroptical phase modulator 140 may apply this adjustment. Furtheradjustments may be determined and applied until the phase offset isminimized, for example to a predetermined threshold.

The receiver system 100A provide efficient recovery of encoded data in anumber of ways. For instance as shown, one OC 130 is used instead of twohybrids in the conventional polarization-diverse and phase-diversereceiver. The number of PDs, TIAs, and ADCs are also reduced by a factorof four as compared to the conventional polarization-diverse andphase-diverse receiver. Further, instead of processing all polarizationand phase information in the electrical domain, which may requirehigh-speed operations, adjustments are made in the optical domain byoptical components, such as the optical polarization controller 110,optical phase modulator 140, and LO 120, which may perform low-speedoperations. Optical components such as optical polarization controller110 and optical phase modulator 140 may also increase sensitivity of thereceiver system without increasing processing power of the DSP 180.Additionally, the feedback control loops 192, 194, 196 may performrecovery of multiple properties simultaneously.

Alternatively or additionally, polarization recovery may be performed bymonitoring the average optical power in a polarization state that isorthogonal to the signal polarization state. For instance, FIG. 1B showsanother example coherent PAM receiver system 100B for receiving a singlepolarization coherent PAM encoded signal. The receiver system 100B isconfigured with similar components as the receiver system 100A of FIG.1A, and is labeled as such. However, additional circuit elements areprovided in receiver system 100B to measure optical power in apolarization that is orthogonal to the polarization of the encodedsignal. For example as shown, the optical polarization controller 110may select the TE-mode polarized signal denoted as x-polarized beam withthe encoded data as output passing through the OC 130, and the TE-modepolarized signal converted from transverse magnetic (TM) mode signaldenoted as y-polarized beam as output passing through a circuitincluding a set of PD 152, TIA 162, and ADC 172, and then to the DSP180. The optical polarization controller 110 shown in FIG. 1B is a 1×2optical polarization controller (1 input, 2 outputs).

To perform polarization recovery, the DSP 180 may maximize the averageoptical power in the x-polarized output by minimizing the averageoptical power in the y-polarized output in a polarization feedbackcontrol loop 198. For instance, initially the DSP 180 may measure anaverage optical power of the y-polarized output from the opticalpolarization controller 110. The DSP 180 may then increase thepolarization control coefficient by a predetermined amount and instructthe optical polarization controller 110, via polarization feedbackcontrol loop 198, to adjust polarization of the coherent PAM encodedsignal by the predetermined amount. After the adjustment, the DSP 180may continue to receive y-polarized output from the optical polarizationcontroller 110, and measure its average optical power. If the DSP 180determines that, after the adjustment, the average optical power of they-polarized output decreased due to the increased polarization controlcoefficient, the DSP 180 may further decrease the polarization controlcoefficient, and the optical polarization controller 110 may apply theadjustment. Conversely, if the DSP 180 determines that the averageoptical power of the y-polarized output increased due to the increasedpolarization control coefficient, the DSP 180 may decrease thepolarization control coefficient, and the optical polarizationcontroller 110 may apply the adjustment. Further adjustments may bedetermined and applied until the average optical power in they-polarization is minimized, for example to a predetermined threshold.

The receiver system 100B provide efficient recovery of encoded data in anumber of ways. For instance, one or more of the PD 152, TIA 162, andADC 172 may be low-speed components, since only average optical power,not encoded data, is analyzed. By performing polarization recovery usingcomponents having a different polarization, polarization recovery forreceiver system 100B may be decoupled from frequency and phase recovery.As such, the receiver system 100B may perform polarization recoverybefore, during, and/or after frequency and/or phase recovery, whichprovides additional flexibility.

In some instances, to increase data capacity, PAM encoded data may beadded to two polarizations, which results in a dual-polarizationcoherent PAM encoded signal. In this regard, FIG. 2A shows an examplecoherent PAM receiver system 200A for receiving a dual-polarizationcoherent PAM encoded signal. As shown, incoming signal may be receivedby the receiver system 200A, which may be modulated by a transmittersystem in both x- and y-polarizations with PAM encoded data. Although x-and y-polarizations are shown in FIG. 2A as the polarization withencoded data, in other examples data may be encoded in any otherpolarizations. The receiver system 200A is configured with similarcomponents as the receiver system 100A of FIG. 1A, and is labeled assuch. For example, similar components, such as OCs, PDs, TIAs, ADCs, LO,DSP, optical phase modulators, are also used in receiver system 200A.However, since the dual-polarization coherent PAM encoded signalincludes data encoded to two different polarizations that carry twoindependent data signals, receiver system 200A includes some differencesfrom the receiver system 100A.

For instance, an optical polarization component, such as a 1×2 opticalpolarization controller, may be provided to separate the receiveddual-polarization coherent PAM encoded signal into two polarized beams.In the example shown in FIG. 2A, an optical polarization demultiplexer210 is used, which is a 1×2 optical controller where the two outputs arealigned to the two polarization multiplexed, orthogonal signals. Theoptical polarization demultiplexer 210 may initially separate thereceived dual-polarization coherent PAM encoded signal into anx-polarized beam and a y-polarized beam, and perform recovery usingfeedback control loops as described further below. These two polarizedbeams may each be analyzed by reference to a signal from an LO 220. Assuch, OC 234 may also separate the LO signal into two LO beams. Then, OC230 mixes the x-polarized beam and the first LO beam, and OC 232 mixesthe y-polarized beam and the second LO beam. The two mixed beams arethen respectively converted from optical to electrical domains byrespective PDs 250, 252, amplified by respective TIAs 260, 262, anddigitized by respective ADCs 270, 272. One or more feedback controlloops may be used between the DSP 280, optical polarizationdemultiplexer 210, LO 220, optical phase modulators 240, 242 to performpolarization, frequency, and/or phase recovery.

With respect to polarization recovery, a low-frequency RF dither signalmay be used. The low-frequency RF dither signal may be added by thetransmitter system (not shown) to either the x-polarized encoded datasignal or the y-polarized encoded data signal to be transmitted. Sincethe RF dither signal was only added to one of the two polarizations, bymaximizing the RF pilot tone of the dither signal in one polarized beam,and/or minimizing the RF pilot tone of the dither signal in the otherpolarized beam, the receiver system 200A may separate the two encodeddata signals. For example, where the dither signal was added by thetransmitter system to the x-polarized signal, then the receiver system200A may perform polarization recovery by maximizing the RF pilot toneof the dither signal in the x-polarized beam; where the dither signalwas added by the transmitter system to the y-polarized signal, then thereceiver system 200A may perform polarization recovery by maximizing theRF pilot tone of the dither signal in the y-polarized beam.

Polarization adjustments may be determined and applied in a polarizationfeedback control loop 292 based on analyzing the pilot tone of the RFdither signal. For instance, the DSP 280 may initially determine the RFpilot tone of the dither signal in the x-polarized beam. The DSP 280 maythen increase the polarization control coefficient by a predeterminedamount and instruct the optical polarization demultiplexer 210, viapolarization feedback control loop 292, to adjust polarization of thedual-polarization coherent PAM encoded signal by the predeterminedamount. After the adjustment, the DSP 280 may continue to receive thedual-polarization coherent PAM encoded signal. If the DSP 280 determinesthat, after the adjustment, the RF pilot tone of the dither signal inthe x-polarized signal increased due to the increased polarizationcontrol coefficient, the DSP 280 may further increase the polarizationcontrol coefficient, and the optical polarization demultiplexer 210 mayapply this adjustment. Conversely, if the DSP 280 determines that the RFpilot tone of the dither signal in the x-polarized signal decreased dueto the increased polarization control coefficient, the DSP 280 maydecrease the polarization control coefficient, and the opticalpolarization demultiplexer 210 may apply this adjustment. Eventually,the RF pilot tone of the dither signal in the x-polarized signal may bemaximized, for example to a predetermined threshold. Additionally oralternatively, the DSP 280 may determine adjustments to minimize the RFpilot tone of the dither signal in the y-polarized signal in thepolarization feedback control loop 292, which may also be applied by theoptical polarization demultiplexer 210.

With respect to carrier frequency and/or phase recovery, one or morefrequency and/or phase feedback control loops may be used between theDSP 280 and the LO 220 and/or optical phase modulator 240, 242. Phaseand/or frequency adjustments to be made may be determined by the DSP 280in the electrical domain, and applied by the LO 220 and/or the opticalphase modulators 240, 242 in the optical domain. For instance, frequencyfeedback control loop 294 may be configured similarly as frequencyfeedback control loop 194 of FIG. 1A. Likewise, frequency and/or phasefeedback control loop 296 may be configured similarly as frequencyand/or phase feedback control loop 196 of FIG. 1A, except that theadjustments to the two LO beams are applied respectively by the opticalphase modulator 240 and the optical phase modulator 242.

The receiver system 200A provides efficient recovery ofdual-polarization encoded data. To receive data encoded to twopolarizations, the number of PDs, TIAs, and ADCs in receiver system 200Aare reduced by a factor of two as compared to a polarization-diverse andphase-diverse receiver system. Further, instead of processing allpolarization and phase information in the electrical domain, which mayrequire high-speed operations, adjustments are made in the opticaldomain by optical components, which may perform low-speed operations.Optical components such as optical polarization demultiplexer 210 andoptical phase modulators 240, 242 may also increase sensitivity of thereceiver system without increasing processing power of the DSP 280.Additionally, the feedback control loops 292, 294, 296 may performrecovery of multiple properties simultaneously.

In some instances, the pilot tone of RF dither signal used forpolarization described above may be monitored by an independentlow-speed circuitry. For instance, FIG. 2B shows another examplecoherent PAM receiver system 200B for receiving a dual-polarizationcoherent PAM encoded signal. The receiver system 200B is configured withsimilar components as the receiver system 200A of FIG. 2A, and islabeled as such. However, an optical tap 212 may be added to divert someof the x-polarized beam to an additional set of PD 254, TIA 264, and ADC274. The diverted x-polarized beam may be used to analyze pilot tone ofRF dither signal inserted into the x-polarized beam, based on whichpolarization may be adjusted as described above. The set of PD 254, TIA264, and ADC 274 may be low-speed components, since only average opticalpower, not encoded signal, is analyzed.

According to some examples, light from multiple lasers of differentfrequencies may be PAM encoded, which results in a coherent PAM encodedsignal with multiple wavelengths, each carrying an independent datasignal. This may result in increased data capacity. FIG. 3A shows anexample coherent PAM receiver system 300A for receiving a coherent PAMencoded signal with multiple wavelengths. As shown, incoming signal maybe received by the receiver system 300A, which may include multiplewavelengths each modulated by a transmitter system with PAM encodeddata. Although four wavelengths are shown in FIG. 3A, which is typicalwhen a coarse wavelength division multiplexing (CWDM) transceiver isused, in other examples data may be encoded in any number ofwavelengths. The receiver system 300A is configured with similarcomponents as the receiver system 100A of FIG. 1A, and is labeled assuch. For example, similar components, such as OCs, PDs, TIAs, ADCs, LO,DSP, optical phase modulators, are also used in receiver system 300A.However, since the coherent PAM encoded signal received by receiversystem 300A includes multiple wavelengths carrying multiple independentdata signals, receiver system 300A includes some differences from thereceiver system 300A.

For instance as shown, the multiple wavelengths may be separated by ademultiplexer into different beams, such as a coarse wavelength divisionmultiplexing (CWDM) demultiplexer (DEMUX) 312. Further as shown, theoptical polarization controller 310 may be placed before the CWDM DEMUX312 in the receiver system 300A. As such, the single opticalpolarization demultiplexer 310 may change polarization of light beforethe different wavelengths are separated into different beams. This alsoallows a single polarization feedback control loop 392 to be used. Oneor more frequency and/or phase feedback control loops, such as frequencyfeedback control loop 394 and frequency and/or phase feedback controlloop 396, may be used between the DSP 380, LO 320, and/or optical phasemodulator 340 to perform frequency and/or phase recovery similarly asdescribed for receiver system 100A. For ease of illustration, OC 330,optical phase modulator 340, PD 350, TIA 360, and ADC 370 are only shownfor one of the wavelengths. However, in actual systems a separate set ofOC, optical phase modulator, PD, TIA, and ADC may be required for eachwavelength of the received PAM encoded signal.

The receiver system 300A is a more efficient multi-wavelength receiverthan a polarization-diverse and phase-diverse receiver system. Althoughthe receiver system 300A requires 4 sets of OCs, optical phasemodulators, PDs, TIAs, and ADCs to receive the additional wavelengths,to receive a same number of wavelengths, a conventionalpolarization-diverse and phase-diverse receiver system would require 16sets of PDs, TIAs, and ADCs, and 8 hybrids. Further, instead ofprocessing all polarization and phase information in the electricaldomain, which may be high-speed operations, adjustments are made by thereceiver system 300A in the optical domain by optical components, whichmay be low-speed operations. Optical components such as opticalpolarization controller 310 and optical phase modulator 340 may alsoincrease sensitivity of the receiver system without increasingprocessing power of the DSP 380. Additionally, the feedback controlloops 392, 394, 396 may perform recovery of multiple propertiessimultaneously.

Further, polarization recovery for PAM encoded data with multiplewavelengths may also be performed by analyzing signals from a separatepolarization. For instance, FIG. 3B shows another example coherent PAMreceiver system 300B for receiving a coherent PAM encoded signal withmultiple wavelengths. The receiver system 300B is configured withsimilar components as the receiver system 300A of FIG. 3A, and islabeled as such. However, referring to FIG. 3B, an optional circuitincluding PD 352, TIA 362, and ADC 372 may be provided to measureoptical power in a polarization that is orthogonal to the polarizationof the data signal. As such, receiver system 300B may performpolarization recovery as described for receiver system 100B. For exampleas shown, DSP 380 may be configured to minimize optical power ofy-polarized light by instructing optical polarization controller 310 viafeedback control loop 398.

Although receiver systems 300A and 300B shown are configured to receivesingle polarization coherent PAM encoded signals, the receiver systems300A and 300B may also be extended to receive dual-polarization PAMencoded signals. For example, the optical polarization controller 310may be replaced with an optical polarization demultiplexer forseparating the two polarizations. Additional optical phase modulators,OCs, PDs, TIAs, and ADCs may be provided for the two polarizations.Recovery of phase and/or frequency may also be performed usingadditional feedback control loops for the two polarizations.

FIG. 4A shows another example receiver system 400A with mixed opticaland electrical signal processing through joint polarization-diverse andphase-diverse detection. Referring to FIG. 4A, the receiver system 400Ais configured with similar components as the receiver system 200A ofFIG. 2A, and is labeled as such. For example, the receiver system 400Aalso includes an LO, a DSP, 3 OCs, and 2 sets of PDs, TIAs, ADCs.However, instead of an optical polarization demultiplexer forpolarization recovery and two optical phase modulators for frequencyand/or phase recovery as shown in FIG. 2A, the receiver system 400Aincludes a 1×2 optical polarization controller 410 configured tointroduce a 45° polarization angle between the received signal afterpolarization adjustment and the LO, and followed by a jointpolarization-diverse and phase-diverse detection circuitry. Thus, only asingle control loop is required to control the 1×2 polarizationcontroller to enable joint polarization-diverse and phase-diversecoherent detection. Final polarization and phase recovery may beachieved in the electrical domain.

For instance as shown, the incoming dual-polarization coherent PAMencoded signal may be represented by vectors A_(in) and B_(in), whereA_(in) and B_(in) are two orthogonally-polarized optical signalscarrying independent data. A 1×2 optical polarization controller 410 maychange the polarization of the incoming dual-polarization coherent PAMencoded signal A_(in) and B_(in), to output A_(out) and B_(out). In thisregard, the 1×2 optical polarization controller 410 may adjust thepolarization of A_(in) and B_(in) such that the resulting A_(out) andB_(out) are still orthogonal to each other, but the resulting A_(out) is45° from a reference axis and aligned with the LO polarization. In thisregard the reference axis may be one of the two principal axes of thePBS labeled as x- and y-axis. For example, A_(out) may be 45° from they-axis in one direction, while B_(out) may be 45° from the y-axis in anopposite direction, making A_(out) 90° from B_(out). As such, the 1×2optical polarization controller 410 may generate two outputs

${X_{out} = {{A_{out\_ x} + B_{out\_ x}} = {{\frac{\sqrt{2}}{2}A_{in}} - {\frac{\sqrt{2}}{2}B_{in}\mspace{14mu}{and}}}}}\mspace{11mu}$$\;{{Y_{out} = {{A_{out\_ y} + B_{out\_ y}} = {{\frac{\sqrt{2}}{2}A_{in}} + {\frac{\sqrt{2}}{2}B_{in}}}}},}$

where A_(out_x) is x component of A_(out), A_(out_y) is y component ofA_(out), B_(out_x) is x component of B_(out) and B_(out_y) is ycomponent of B_(out).

Further as shown, the LO signal from LO 420 may be split by OC 434 intotwo signals, LO1 and LO2. OC 434 may introduce a n/2 phase differencebetween LO1 and LO2. For example, a 3 dB OC that separates a beam usinga cross path and a parallel path may have a n/2 phase difference betweenthe two paths. Then, OC 430 may mix X_(out) with LO1, and generate twooutputs C_(x+)=½A_(in)−½B_(in)+j½E_(L), andC_(x−)=½A_(in)−½jB_(in)+½E_(L), where E_(L) denotes the complex opticalfield of LO1, and jE_(L) denotes the complex field of LO2. Likewise, OC432 may mix the Y_(out) with LO2, and generate two outputsC_(y+)=½A_(in)+½B_(in)−½E_(L), and C_(y−)=½jA_(in)+½jB_(in)+½jE_(L).

Outputs from OC 430 and OC 432 are then respectively passed through PDs450, 452 for optical to electrical conversion, TIAs 460, 462 foramplification, and ADCs 470, 472 for electrical to digital conversion,resulting in two digitized electrical signals. The two digitizedelectrical signals may be represented by D_(x)=A_(in)E_(L) sin(φ_(x))−B_(in)E_(L) sin (φ_(y)), and D_(y)=A_(in)E_(L) cos(φ_(x))+B_(in)E_(L) cos (φ_(y)), where φ_(x) is the relative carrierphase difference between the received signal A_(out) and the LO, whileφ_(y) is the relative carrier phase difference between the receivedsignal B_(out) and the LO.

DSP 480 may determine the carrier phase differences using pilot datasymbol based carrier phase estimation algorithms. For instance, twoknown PAM pilot data symbols, such as d1 and d2, may be inserted intothe two orthogonally-polarized incoming coherent PAM encoded signal atdifferent time slots. For example, d1 may be inserted in onepolarization channel, for example channel for A_(in), at time slot n,while d2 may be inserted in another polarization channel, for examplechannel for B_(in) at time slot n+1. The received digitized pilot datasymbol of d1 may then be given by D_(1(n)x)=A_(in)E_(L) sin (φ_(x)), andD_(1(n)y)=A_(in)E_(L) cos (φ_(x)). The DSP 480 can then solve the twoequations to extract φ_(x). The digitized pilot data symbol of d2 can begiven by D_(2(n+1)x)=−B_(in)E_(L) sin (φ_(y)), andD_(2(n+1)y)=B_(in)E_(L) cos (φ_(y)). The DSP 480 can then solve the twoequations to extract φ_(y).

Further, DSP 480 may also determine polarization information of theincoming signals using the pilot data symbol. For instance, once thevalues of φ_(x) and φ_(y) are obtained, A_(in) and B_(in) can bedetermined by the DSP 480 based on the above relationshipsD_(x)=A_(in)E_(L) sin (φ_(x))−B_(in)E_(L) sin (φ_(y)), andD_(y)=A_(in)E_(L) cos (φ_(x))+B_(in)E_(L) cos (φ_(y)). Then, DSP 480 maydetermine polarization states of A_(out) and B_(out) based on A_(m) andB_(in), and the relationships above for the digitized pilot data symbolsD_(1(n)x), D_(1(n)y), D_(2(n+1)x), D_(2(n+1)y). Further, since thecurrent polarization adjustments applied by the 1×2 optical polarizationcontroller 410 to the A_(in) and B_(in) are also known, polarizationstates of A_(in) and B_(in) may be deduced from the polarization ofA_(out) and B_(out). Since polarization of the incoming signals maychange randomly, dynamic adjustments may need to be applied to maintaina 45° difference between A_(out) and the reference axis.

As such, the current polarization states of the incoming signal A_(in)and B_(in) are determined, the DSP 480 may determine the appropriateadjustments to be made by the 1×2 optical polarization controller 410via polarization control loop 492. For instance, DSP 480 may determinepolarization offsets between A_(in) (and/or B_(in)) and the LO signal,and determine adjustments to be made to A_(in) (and/or B_(in)) to ensurethat A_(out) have a 45° offset from the reference y-axis, which wouldtypically be aligned with the polarization of the LO. The purpose tohave a 45° polarization angle between A_(out) (and/or B_(out)) and theLO is to make sure that the signal components of A_(out) and B_(out) canbe equally distributed over the two output ports X_(out) and Y_(out) ofthe 1×2 polarization controller. As such, half of X_(out) (or Y_(out))comes from A_(out) and another half comes from B_(out). As shown by theformulae above, such a two-port equal signal components distributioncould enable joint polarization-diverse and phase-diverse coherentdetection, in which the polarization diverse detection circuitry and thephase diverse detection circuitry are combined into a single circuitry,which includes the 1×2 optical polarization controller 410, two opticalcouplers 430, 432, two sets of PDs 450, 452, TIAs 460, 462 and ADCs 470,472, as well as the DSP 480. In contrast, with conventionalpolarization-diverse and phase-diverse coherent detection, in additionto the DSP, two hybrids plus four sets of PDs, TIAs and ADCs are needed.

Since relatively accurate incoming signal polarization and phaseinformation can be extracted from the pilot data symbol as analyzedusing the relationships above, only one polarization adjustment by the1×2 optical polarization controller 410 may be needed for each change ofincoming signal polarization through the control loop 492. This couldimprove the response time of the polarization controller as compared tothe conventional trial and error based feedback control algorithms,where typically multiple polarization adjustments are needed for eachpolarization change. Note that here the polarization control loop is notused to align the received signal polarization to the LO in the opticaldomain like the receiver systems shown in FIGS. 1A-3B. Instead, thepolarization control loop 492 is used to enable jointpolarization-diverse and phase-diverse coherent detection. For thismethod, the final polarization and phase recovery are achieved in theelectrical domain by the DSP 480 using the relationships describedabove. Furthermore, in the feedforward based polarization and phaserecovery algorithms, polarization and phase errors can be estimated andremoved from the current data symbols without feedback delay, which mayimprove overall polarization and phase tracking speed. Especially, theuse of fast feedforward based phase recovery algorithms could improvethe laser phase noise tolerance.

The receiver system 400A is a more efficient multi-wavelength receiverthan a polarization-diverse and phase-diverse receiver system. Forinstance, the receiver system 400A requires 2 sets of PDs, TIAs, andADCs, as compared to the 4 sets required by conventionalpolarization-diverse and phase-diverse receiver system. The receiversystem 400A also uses only one optical polarization controller, insteadof two hybrids in the conventional polarization-diverse andphase-diverse receiver system, or the two optical phase modulators inthe receiver system 200A. Further, instead of processing allpolarization and phase information in the electrical domain, which maybe high-speed operations, adjustments are made by the receiver system400A in the optical domain by optical components, which may be low-speedoperations. Optical components such as the 1×2 optical polarizationcontroller 410 may also increase sensitivity of the receiver systemwithout increasing processing power of the DSP 480. Additionally, thesingle control circuitry or loop allows recovery of multiple properties,which may be performed simultaneously.

Further, polarization recovery for the example system with jointpolarization- and phase-diverse detection may also be performed byanalyzing signals from a separate polarization. For instance, FIG. 4Bshows another example coherent PAM receiver system 400B with jointpolarization- and phase-diverse detection. The receiver system 400B isconfigured with similar components as the receiver system 400A of FIG.4A, and is labeled as such. However, referring to FIG. 4B, an optionalcircuit including PD 454, TIA 464, and ADC 474 may be provided tomeasure pilot tone of dither signals in the incoming signals. As such,receiver system 400B may perform polarization recovery similarly asdescribed for receiver system 200B. For example as shown, polarizationoffsets may be monitored by inserting two low-speed dither signals intothe two orthogonally-polarized channels for A_(in) and B_(in), and thepilot tones may then be monitored by the DSP 480 using low-speedcomponents including optical tap 412, PD 454, TIA 464, ADC 474. Theintroduction of this additional low-speed polarization control loopenables independent polarization adjustment of the 1×2 polarizationcontroller 410 without performing carrier phase recovery and timingsynchronization, which may simplify the receiver control loop design.

Although FIGS. 4A and 4B show the example receiver systems 400A and 400Bconfigured for receiving a dual-polarization coherent PAM encoded signalwith a single wavelength, features of the receiver system may beextended for receiving single polarization signals and signals withmultiple wavelengths. For instance, for receiving single polarizationcoherent PAM encoded signal, the receiver system 400A may perform thesame operations, but assuming either A_(in) or B_(in) to be zero. Forreceiving a coherent PAM encoded signal with multiple wavelengths, awavelength demultiplexer such as a CWDM demultiplexer may be addedsimilarly as shown in FIG. 3A.

FIGS. 5A, 5B, and 5C show example implementations for a 1×2 opticalpolarization controller. Referring to FIG. 5A, 1×2 optical polarizationcontroller 500A includes a PBS 510 that splits the incomingdual-polarization coherent PAM encoded signal including A_(in) andB_(in) into two beams X_(in) and Y_(in). Then, phase shifter 520 makesphase adjustments to one of the split beams. For example as shown, phaseof the beam X_(in) is changed by phase shifter 520 based on adjustmentsdetermined by the DSP 480. The adjusted beam X_(out) and unadjusted beamY_(in) may then pass through OC 530 for further processing. Here thereceived signal polarization is adjusted by changing the relative phasebetween its X-component and Y-component. The relative phase adjustmentcan be made by only adjusting the phase for the X-component or byadjusting the phase for both the X-component and the Y-component. Lessvoltage applied to the phase shifter is required if phase adjustmentsare made to both the X- and Y-components.

Referring to FIG. 5B, 1×2 optical polarization controller 500B includessimilar components as 1×2 optical polarization controller 500A, and islabeled as such. However, the 1×2 optical polarization controller 500Bfurther includes a second phase shifter 540. The second phase shifter540 may change phase of the beam Y_(in) based on adjustments determinedby DSP 480. The adjusted beams X_(out) and Y_(out) may then pass throughOC 530 for further processing. For example as shown, a second-stage 2×2MZI with phase shifter 550 may be used. The second stage 2×2 MZI 550 maybe used to further reduce the required control voltage for each phaseshifter. It may also be used to enable IM-DD PAM and coherent PAMtwo-mode operation as described below, where all the signal componentsof A_(in) (or B_(in)) are directed to one of the two output ports of the1×2 polarization controller.

Referring to FIG. 5C, 1×2 optical polarization controller 500C includessimilar components as 1×2 optical polarization controller 500B, and islabeled as such. However, the 1×2 optical polarization controller 500Cis further configured to receive a coherent PAM encoded signal withmultiple wavelengths. In this regard, one or more wavelengthdemultiplexers, such as CWDM DEMUX 560 and CWDM DEMUX 562, are provided.As shown, CWDM DEMUX 560 splits the various wavelengths in beam X_(in)into separate beams, and CWDM DEMUX 562 splits the various wavelengthsin beam Y_(in) into separate beams. The multiple wavelengths may sharethe same PBS 510 and CWDM DEMUX 560 and 562, but may not share the samephase shifters, OC, and second-stage 2×2 MZI with phase shifter. Thus,although not shown, a separate set of phase shifters, OC, andsecond-stage 2×2 MZI with phase shifter may be required for eachwavelength received.

Instead of or in addition to mixed optical and electrical processing, acoherent PAM receiver may be provided with mixed analog and digitalprocessing. For instance, FIG. 6 shows an example coherent PAM receiverwith mixed analog and digital processing for receiving dual-polarizationcoherent PAM encoded signal. As shown, the receiver system 600 includessome similar components as a polarization-diverse and phase-diverse,such as PBS 610, LO 620, OC 630, two hybrids 640, 642, four sets of PDs650, 652, 654, 656, and TIAs 660, 662, 664, 666, and DSP 680. However,to perform mixed analog and digital processing, receiver system 600includes some differences from a polarization-diverse and phase-diversereceiver system.

For instance, the receiver system 600 further includes an analogequalizer 690 for performing both polarization and carrier phaserecovery. As an example, the analog equalizer 690 may be a 4×2 analogmultiple-input-multiple-output (MIMO) equalizer. To perform polarizationand carrier phase recovery, the analog equalizer 690 may receive fourinputs from the four respective PDs/TIAs, which may include detectedin-phase component in x-polarization (Ix), detected quadrature componentin x-polarization (Qx), detected in-phase component in y-polarization(Iy), and quadrature component in y-polarization (Qy). Note thatalthough PAM signal has no quadrature components, the carrier hasquadrature components. Unless the carrier phase is removed and thesignal and LO polarization is aligned prior to coherent mixing as shownin FIG. 1A-3B, polarization-diverse and phase-diverse coherent detectioncircuitry is needed. Based on these inputs, the analog equalizer 590 maygenerate analog outputs to the DSP 680. For instance, the analogequalizer 590 may generate two polarization and phase recoveredreal-valued signals, Ix_output and Iy_output, where Ix_output denotesthe PAM signal carried over the X-polarization channel while Iy_outputdenotes the PAM signal carried over the Y-polarization channel. Forexample, Ix_output and Iy_output may be generated based on therelationships Ix_output=C₁Ix+C₂Qx+C₃Iy+C₄Qy, andIy_output=D₁Ix+D₂Qx+D₃Iy+D₄Qy, where C_(n) and D_(m) are equalizercoefficients. The outputs may be converted into digital outputs by ADCs670 and 672 before being passed to DSP 680.

The coefficients to be used by the analog equalizer 690 may bedetermined by the DSP 680 using one or more control loops, such asfeedback control loop 692. In this regard, the DSP 680 may make variousdeterminations in the digital domain, while the analog equalizer 690 maymake various determinations in the analog domain. For instance,initially the analog equalizer 690 may detect the Ix, Qx, Iy and Qy, andcalculate Ix_output and Iy_output using predetermined coefficients. Theanalog equalizer 690 may then send the Ix_output and Iy_output to theDSP 680. Based on the Ix_output and the Iy_output, the DSP 580 maydetermine new coefficients to be used by the analog equalizer 690, andinstruct the analog equalizer 690 via feedback control loop 692. Theanalog equalizer coefficients may be obtained by the DSP 680 bycomparing the two outputs of the analog equalizer 690 with the expectedPAM signal level distributions using classic gradient descent basedalgorithms such as the well known least mean square (LMS) algorithm. Theanalog equalizer 690 may then continue to detect the Ix, Qx, Iy and Qy,and use the new coefficients to calculate another set of Ix_output andIy_output, which are again sent to the DSP 680. Further coefficients maybe determined by the DSP 680 and used by the analog equalizer 690 untilthe calculated mean square error through the LMS algorithm is below apredetermined threshold or reach its minimum. Thus, the feedback controlloops 692 enables both polarization and phase recovery, which may alsobe simultaneous.

The receiver system 600 provide efficient recovery of encoded data usingmixed analog and digital processing. For instance, by using the 4×2analog MIMO equalizer, the required number of ADCs are reduced by afactor of two as compared to the receiver system 100, which results inpower saving. The receiver system 600 with mixed analog and digitalsignal processing may also be extended for receiving a coherent PAMencoded signal with multiple wavelengths. For instance, a mixed analogand digital processing receiver for 4 wavelengths may require 8 hybridsand 16 sets of PDs and TIAs, but only 8 ADCs, unlike apolarization-diverse and phase-diverse receiver system that wouldrequire 16 ADCs for receiving 4 wavelengths. Thus, the receiver system600 is also a more efficient multi-wavelength receiver.

Although the example receiver systems described above include eithermixed optical/electrical signal processing, or mixed analog/digitalsignal processing, other combinations may be used. For example, areceiver system may include mixed optical and analog signal processing,where functions of the DSP is implemented using analog circuits, whichmay further reduce power consumption. As another example, a receiversystem may include mixed optical/analog/digital signal processing, wheresome functions of the DSP are moved to analog circuits while others areperformed digitally, which may also reduce power consumption of the DSP.

Although the example receiver systems described above are shown toprocess coherent PAM encoded signals, each of the example receiversystems may be further configured with compatibility to IM-DD PAMencoded signals. In this regard, the receiver systems described abovemay be provided with two modes—a first mode for receiving coherent PAMencoded signals, and a second mode for receiving IM-DD PAM encodedsignals. For instance, if signals are received from a coherent PAMtransmitter system, the receiver system may be configured in the firstmode; and if signals are received from an IM-DD PAM transmitter system,the receiver system may be configured in the second mode. FIGS. 7A and7B show an example transmission system where the receiver systemperforms mixed signal processing. In FIG. 7A, the transmission system700A is configured to communicate using a coherent PAM encoded signal.In FIG. 7B, the transmission system 700B is configured to communicateusing an IM-DD PAM encoded signal.

Referring to FIG. 7A, the coherent PAM transmission system 700A includesa coherent PAM transmitter system 710 and a coherent PAM receiver system720. Although four lasers with four wavelengths are shown in FIG. 7A, inother examples data may be encoded in any number of wavelengths. Forease of illustration, operations involving only one wavelength is shown(as lines and arrows), but it should be understood that analogousoperations may be performed for each of the wavelengths. Further,components such as 2-mode switches 740 and 770, MZM 750, optical phasemodulator 340, CWDM DEMUX 312, CWDM MUX 760, PD 350, TIA 360, and ADC370 are only shown for one of the wavelengths. However, a separate setof 2-mode switches, MZM, optical phase modulator, CWDM MUX and DEMUX,PD, TIA, and ADC may be required for each wavelength of the received PAMencoded signal. Still further, control loops for recovery ofpolarization, phase, and/or frequency are also omitted.

The coherent PAM transmitter system 710 may encode data on coherentcarrier light for transmission. For example, the coherent PAMtransmitter system 710 may include one or more lasers, such as lasers732, 734, 736, 738 shown, which may emit light of different fourwavelengths. The emitted light from each laser may be passed through aswitch, such as 2-mode switch 740. In this regard, the 2-mode switch 740may be set to a first mode for transmitting coherent PAM signals. Inthis first mode, the 2-mode switch essentially acts as an OC, whichallows part of the emitted light to be PAM encoded by a data modulator,such as an MZM 750, while also diverts part of the emitted light to thereceiver system 720 to be used as an LO signal. The 2-mode switch may beimplemented using a silicon photonics based Mach-Zehnder interferometer.For example, the 2-mode switch 740 may be a 2×2 Mach-Zehnderinterferometer (MZI), in which the operation mode can be switched byadjusting the relative phase between the two paths of the MZI. Furtheras shown, the encoded light from the different lasers may then becombined by a wavelength multiplexer, such as a CWDM MUX 760. Thecoherent PAM encoded signal with multiple wavelengths may then betransmitted to the coherent PAM receiver system 720, for example via oneor more optical fibers.

The coherent PAM receiver system 720 may then receive the coherent PAMencoded signal with multiple wavelengths from the transmitter system710. In this example, the coherent PAM receiver system 720 is configuredwith similar components as the receiver system 300A of FIG. 3A, and islabeled as such. However, to provide backward interoperability, thecoherent PAM receiver system 720 includes some differences from thereceiver system 300A. For instance, OC 330 of receiver system 300A isreplaced with 2-mode switch 770. To receive coherent PAM encoded signal,the 2-mode switch 770 may be set in a first mode, which essentially actsas the OC 330 of receiver system 300A. Like the 2-mode switch 740, the2-mode switch 770 may also be implemented using a silicon photonicsbased Mach-Zehnder interferometer, such as a 2×2 MZI. Thus in the firstmode, the receiver system 720 is configured the same way as the receiversystem 300A of FIG. 3A.

The coherent PAM transmission system 700A may be re-configured into anIM-DD PAM transmission system. For instance, referring to FIG. 7B, theIM-DD PAM transmission system 700B includes an IM-DD PAM transmittersystem 712 and an IM-DD PAM receiver system 722. As shown, the IM-DD PAMtransmitter system 712 is configured with similar components as thecoherent PAM transmitter system 710 of FIG. 7A, and the IM-DD PAMreceiver system 722 is configured with similar components as thecoherent PAM receiver system 720 of FIG. 7A. Further, to ensurecompatibility, the number of lasers in the IM-DD PAM transmission system700B remains the same as the coherent PAM transmission system 700A.However, to perform IM-DD PAM transmission, IM-DD PAM transmissionsystem 700B includes some differences from the coherent PAM transmissionsystem 700A.

For instance, in the IM-DD PAM transmitter system 712, the 2-mode switch740 is set to a second mode. Since only intensity is modulated for IM-DDPAM, there is no need for an LO signal as reference. As such, in thesecond mode the 2-mode switch 740 may direct all laser power from alaser to the signal path of the IM-DD PAM receiver system 722. Thus asshown, all laser power from the laser 732 is directed by the 2-modeswitch 740 to be modulated by a data modulator, which may also be an MZM750. Thus, to change from coherent PAM transmission to IM-DD PAMtransmission, all that is required is to change the mode for 2-modeswitch 740 and the modulation performed by the MZM 750.

Likewise, in the IM-DD PAM receiver system 722, the 2-mode switch 770 isset to a second mode. Since data was only encoded by intensity for IM-DDPAM, there is no need to perform polarization, frequency, and/or phaserecovery with a LO signal. As such, in the second mode the 2-mode switch770 may act as a switch that receives all laser power for eachwavelength. For instance, the received laser power for one wavelengthmay then be converted from optical to electrical domain by PD 350,amplified by TIA 360, digitized by ADC 370, and then analyzed anddecoded by DSP 380. Further, since only total intensity is measured,there is no need for canceling common-mode noise. Thus, where a pair ofbalanced PDs are used, in the second mode the 2-mode switch 770 maydirect all received optical power for each wavelength to one of the PDs.

Although the transmission systems 700A and 700B are shown with receiversystems 720 and 722 configured similarly as receiver system 300A, thereceiver systems may alternatively configured as any of the receiversystems described in FIGS. 1A-4B, and 6. For example, to receivedual-polarization coherent PAM encoded signal, the optical polarizationcontroller 310 may be replaced with an optical polarizationdemultiplexer as in receiver system 200A or 200B, or by a 2×1 opticalpolarization controller as in receiver system 400A or 400B. Additionaloptical phase modulators, OCs, PDs, TIAs, and ADCs may be provided forthe two polarizations. As another example, the receiver system intransmission systems 700A and 700B may be configured with mixed analogand digital processing, such as shown in FIG. 6.

FIG. 8 illustrates an example block diagram of some components in areceiver system, such as receiver system 100A, 100B, 200A, 200B, 300A,300B, 400A, 400B, or 600. It should not be considered as limiting thescope of the disclosure or usefulness of the features described herein.In this example, the receiver system is shown with one or more computingdevices 810. The computing devices 810 contains one or more processors820, memory 830 and other components typically present in generalpurpose computing devices. Memory 830 of the computing devices 810 canstore information accessible by the one or more processors 820,including instructions 834 that can be executed by the one or moreprocessors 820.

Memory 830 can also include data 832 that can be retrieved, manipulatedor stored by the processor. The memory can be of any non-transitory typecapable of storing information accessible by the processor, such as ahard-drive, memory card, ROM, RAM, DVD, CD-ROM, write-capable, andread-only memories. For example, the data 832 may include parameters,thresholds, and other values for performing polarization, frequency,and/or phase recovery.

Data 832 may be retrieved, stored, or modified by the one or moreprocessors 820 in accordance with the instructions 834. For instance,although the subject matter described herein is not limited by anyparticular data structure, the data can be stored in computer registers,in a relational database as a table having many different fields andrecords, or XML documents. The data can also be formatted in anycomputing device-readable format such as, but not limited to, binaryvalues, ASCII or Unicode. Moreover, the data can comprise anyinformation sufficient to identify the relevant information, such asnumbers, descriptive text, propriety codes, pointers, references to datastored in other memories such as at other network locations, orinformation that is used by a function to calculate the relevant data.

The instructions 834 can be any set of instructions to be executeddirectly, such as machine code, or indirectly, such as scripts, by theone or more processors. In that regard, the terms “instructions,”“application,” “steps,” and “programs” can be used interchangeablyherein. The instructions can be stored in object code format for directprocessing by a processor, or in any other computing device languageincluding scripts or collections of independent source code modules thatare interpreted on demand or compiled in advance. For instance, theinstructions 834 may include functions or methods for performingpolarization, frequency, and/or phase recovery.

The one or more processors 820 can be any conventional processors, suchas a commercially available CPU. Alternatively, the processors can bededicated components such as an application specific integrated circuit(“ASIC”) or other hardware-based processor. For example, DSP 180, 280,380, 480, and 680 may be configured as the one or more processors 820,with access to memory such as data and instructions. Although notnecessary, one or more of the computing devices 810 may includespecialized hardware components to perform specific computing processes.

Although FIG. 8 functionally illustrates the processor, memory, andother elements of computing devices 810 as being within the same block,the processor, computer, computing device, or memory can actuallycomprise multiple processors, computers, computing devices, or memoriesthat may or may not be stored within the same physical housing. Forexample, the memory can be a hard drive or other storage media locatedin housings different from that of the computing devices 810.Accordingly, references to a processor, computer, computing device, ormemory will be understood to include references to a collection ofprocessors, computers, computing devices, or memories that may or maynot operate in parallel. For example, the computing devices 810 mayinclude server computing devices operating as a load-balanced serverfarm, distributed system, etc. Yet further, although some functionsdescribed below are indicated as taking place on a single computingdevice having a single processor, various aspects of the subject matterdescribed herein can be implemented by a plurality of computing devices,for example, communicating information over a network.

The computing devices 810 may be capable of directly and indirectlycommunicating with other transmitter systems and/or receiver systemsover a network. Computing devices in a network, such as computingdevices 810, may be interconnected using various protocols and systems,such that computing devices in the network can be part of the Internet,World Wide Web, specific intranets, wide area networks, or localnetworks. Computing devices in the network can utilize standardcommunication protocols, such as Ethernet, WiFi and HTTP, protocols thatare proprietary to one or more companies, and various combinations ofthe foregoing. Although certain advantages are obtained when informationis transmitted or received as noted above, other aspects of the subjectmatter described herein are not limited to any particular manner oftransmission of information.

Example Methods

Further to example systems described above, example methods are nowdescribed. Such methods may be performed using the systems describedabove, modifications thereof, or any of a variety of systems havingdifferent configurations. It should be understood that the operationsinvolved in the following methods need not be performed in the preciseorder described. Rather, various operations may be handled in adifferent order or simultaneously, and operations may be added oromitted.

FIG. 9 shows an example flow diagram 800 for receiving coherent PAMencoded signal. Flow diagram 900 may be performed by a receiver system,such as any of the receiver systems shown in FIGS. 1A-4, and 6.Processors in the receiver system, such as processors 820 of FIG. 8, mayreceive data and make various determinations as shown in the flowdiagram 900.

Referring to FIG. 9, at block 910, a coherent Pulse Amplitude Modulation(PAM) encoded signal may be received. For example, the coherent PAMencoded signal may be a single polarization signal, such as shown inFIGS. 1A, 1B, 3A, and 3B. As another example, the coherent PAM encodedsignal may be a dual-polarization signal, such as shown in FIGS. 2A, 2B,4, and 6. As still another example, the coherent PAM encoded signal mayinclude multiple wavelengths, such as shown in FIGS. 3A and 3B.

At block 920, a local oscillator (LO) signal is received. For instance,for single polarization PAM encoded signal with a single wavelength asshown in FIGS. 1A and 1B, the LO signal may be received and mixed withone PAM signal. In contrast, for dual-polarization PAM encoded signalsuch as shown in FIGS. 2A, 2B, 4, 6, or for PAM encoded signal withmultiple wavelengths such as shown in FIGS. 3A and 3B, the LO signalreceived may be split into multiple beams before being combining withrespective PAM encoded beams of different polarization and/orwavelengths.

At block 930, polarization recovery between the received coherent PAMencoded signal and the LO signal is performed using a first control loopusing mixed signal processing. For instance as described in relation toFIGS. 1A and 3A, the polarization feedback control loop may be performedby analyzing beating signal between LO and a polarized portion of thereceived coherent PAM encoded signal with encoded data. Alternatively asdescribed in relation to FIGS. 1B and 3B, the polarization feedbackcontrol loop may be performed by minimizing optical power of a polarizedportion of the received coherent PAM encoded signal without encodeddata. In another example described in relation to FIGS. 2A and 2B,polarization feedback control loop may be performed by minimizing pilottone of a dither signal. As another alternative shown in FIGS. 4A and4B, the polarization control loop may include a 1×2 polarizationcontroller followed by a joint polarization-diverse and phase-diversecoherent detection circuitry. The 1×2 polarization controller may beconfigured to introduce a 45° polarization angle between the receivedsignal after polarization adjustment and the LO, and the finalpolarization recovery may be achieved in the electrical domain. This isdifferent from the polarization recovery methods described in FIGS.1A-3B, where polarization controller/demultiplexer is introduced toalign the received signal polarization to the LO such that polarizationrecovery can be achieved in the optical domain while electricalprocessing is only used to determine how to adjust the polarizationcontroller/demultiplexer.

As still another example described in relation to FIG. 6, polarizationrecovery may be performed with mixed analog and digital signalprocessing via a feedback control loop between an analog equalizer 690and a DSP 680.

At block 940, phase recovery between the received coherent PAM encodedsignal and the LO signal using a second control loop using mixed signalprocessing. For example as described in relation to FIGS. 1A, 1B, 2A,2B, 3A, 3B, the phase feedback control loop may be performed byanalyzing pilot data symbols. As an alternative, a feedforward controlloop may be used to perform phase recovery as shown in FIGS. 4A and 4B.Further as described in relation to FIG. 6, phase recovery may beperformed with mixed analog and digital signal processing via a feedbackcontrol loop between an analog equalizer 690 and a DSP 680.Additionally, for instance as described in relation to FIGS. 1A, 1B, 2A,2B, 2A, and 3B, a frequency feedback control loop may be performed byanalyzing FFT between the LO signal and the coherent PAM encoded signal.

The technology provides power efficient receiver systems in datacenters.The technology provides receiver systems that are compatible with bothcoherent PAM and IM-DD PAM transmission systems, which increases designflexibility of a datacenter. Further, by using mixed optical andelectrical processing, and/or mixed electrical and digital processing,components with high power consumption may be reduced. Additionally, byusing a PAM system that does not perform phase modulation, requirementson optical equipment, such as lasers, may be relaxed, which may furtherincrease power efficiency and design flexibility.

Unless otherwise stated, the foregoing alternative examples are notmutually exclusive, but may be implemented in various combinations toachieve unique advantages. As these and other variations andcombinations of the features discussed above can be utilized withoutdeparting from the subject matter defined by the claims, the foregoingdescription of the embodiments should be taken by way of illustrationrather than by way of limitation of the subject matter defined by theclaims. In addition, the provision of the examples described herein, aswell as clauses phrased as “such as,” “including” and the like, shouldnot be interpreted as limiting the subject matter of the claims to thespecific examples; rather, the examples are intended to illustrate onlyone of many possible embodiments. Further, the same reference numbers indifferent drawings can identify the same or similar elements.

We claim:
 1. A receiver system, comprising: a polarization beam splitterconfigured to split a received coherent Pulse Amplitude Modulation (PAM)encoded signal into two polarized PAM encoded signals; two hybridsconfigured to combine each of the two polarized PAM encoded signals witha local oscillator (LO) signal; an analog equalizer configured to:perform polarization recovery between the received coherent PAM encodedsignal and the LO signal; perform phase recovery between the receivedcoherent PAM encoded signal and the LO signal; and a digital signalprocessor (DSP) configured to provide equalizer coefficients to theanalog equalizer for polarization and phase recovery.
 2. The receiversystem of claim 1, wherein the analog equalizer is a 4×2 multi-inputmulti-output equalizer.
 3. The receiver system of claim 1, wherein theanalog equalizer is further configured to receive four input signals. 4.The receiver system of claim 3, wherein the four input signals include adetected in-phase component in x-polarization (Ix), a detectedquadrature component in x-polarization (Qx), a detected in-phasecomponent in y-polarization (Iy), and a quadrature component iny-polarization (Qy).
 5. The receiver system of claim 3, wherein theanalog equalizer receives the four input signals from fourphotodetectors and transimpedance amplifiers.
 6. The receiver system ofclaim 1, wherein the equalizer coefficients are determined by the DSPusing one or more control loops.
 7. The receiver system of claim 6,wherein at least one of the one or more control loops is a feedbackcontrol loop.
 8. The receiver system of claim 1, wherein the equalizercoefficients are determined by the DSP by comparing two outputs of theanalog equalizer with an expected PAM signal level distribution.
 9. Thereceiver system of claim 9, wherein comparing the two outputs of theanalog equalizer with the expected PAM signal level distributionincludes using a gradient descent based algorithm.
 10. The receiversystem of claim 9, wherein the gradient descent based algorithm is aleast mean square algorithm.
 11. A method, comprising: receiving, by areceiver system, a coherent Pulse Amplitude Modulation (PAM) encodedsignal; receiving, by the receiver system, a local oscillator (LO)signal; performing, by the receiver system using mixed signalprocessing, polarization recovery between the received coherent PAMencoded signal and the LO signal using a first control loop; andperforming, by the receiver system using mixed signal processing, phaserecovery between the received coherent PAM encoded signal and the LOsignal using a second control loop.
 12. The method of claim 11, whereinthe mixed signal processing includes processing in an optical domain andprocessing in an electrical domain.
 13. The method of claim 11, whereinthe mixed signal processing includes processing in an analog domain andprocessing in a digital domain.
 14. The method of claim 11, furthercomprising receiving, by the receiver system from a digital signalprocessor (DSP), equalizer coefficients for the polarization and phaserecovery.
 15. The method of claim 14, further comprising determining, bythe DSP, the equalizer coefficients using one or more control loops. 16.The method of claim 15, wherein at least one of the one or more controlsloops is a feedback control loop.
 17. The method of claim 14, furthercomprising determining, by the DSP, the equalizer coefficients bycomparing two outputs of the analog equalizer with an expected PAMsignal level distribution.
 18. The method of claim 17, wherein comparingthe two outputs of the analog equalizer with the expected PAM signallevel distribution includes using a gradient descent based algorithm.19. The method of claim 11, wherein when performing the polarization andphase recovery, the method further comprises receiving, by the receiversystem, four input signals.
 20. The method of claim 18, wherein the fourinput signals include a detected in-phase component in x-polarization(Ix), a detected quadrature component in x-polarization (Qx), a detectedin-phase component in y-polarization (Iy), and a quadrature component iny-polarization (Qy).